When you say remote tnsnames.ora do you mean the database you are connecting to with the DB link or the database you are connecting from

If it is the database you are trying to connect to having the tnsnames entry won't help, you need it on the server you are connecting from.

EX.
DB1 on server1 has a db link that connects to DB2 on server2

The tnsnames entry for DB2 has to be in server1's tnsnames.ora file

A good test is to add the entry and log onto the server. Once on the server try to connect using SQL plus to the database you want the db link to connect to.

Hope this helps